The ancient Earth has seen its global climate come and go — a few million years of ice-covered cold at times, and long epochs of dry, ice-free warmth at others, all due to the vagaries of nature.

Then there are danger times like today, when after thousands of years of relatively low temperatures around the globe, planet Earth is warming measurably as greenhouse gases of the industrial age spur an ever-faster rise in worldwide thermometer readings.

A University of California, Davis scientist and other researchers examining the past for clues to the present are suggesting that global warming and cooling have run in cycles. They've uncovered evidence that 300 million years ago a long-lasting ice age froze entire continents and then gave way 40 million years ago to a period of global warming that melted all the ice and left the Earth dry, dust-blown and covered only with sparse vegetation.

During those extreme temperature swings, said Isabel P. Montanez, a geochemist and geology professor at UC Davis, came shorter periods of warming and cooling.

It was a highly unstable period of climate change, marked by millions of years when temperatures yo-yoed up and down as the atmosphere's natural levels of carbon dioxide, the major heat-trapping greenhouse gas, rose and fell wildly, says Montanez.

Montanez led a team of researchers that gathered evidence of fluctuations in ancient carbon dioxide levels by analyzing fossil plant leaves and weathered rocks throughout the American Southwest, ice cores in Antarctica, Australian fossils, and coal formations in China.

By recording species changes in long-vanished, shallow-water creatures called brachiopods, the scientists calculated how sea levels rose during warming periods as glaciers and continental ice sheets disappeared.

Unstable phenomenon

The team's conclusions are published in the journal Science. In an interview Thursday, Montanez said the report is the first well-documented study showing the transformation of a long-lasting ice age into an even longer period of increasing global warmth.

Surface temperatures from the Paleozoic ice age and the warm period that followed left their signs in the varied species of brachiopods, she said. And the advances and retreats of massive glaciers in Australia provided signs in scoured mountainsides that the global warming phenomenon was highly unstable.

The ancient supercontinent of 300 million years ago that left its evidence for the team to study is known as Gondwana, and most of today's Southern Hemisphere continents, from Australia to Africa and South America, were formed when Gondwana broke up.

Continent began to thaw

During the long Paleozoic ice age, vast continental ice sheets were many miles thick on southern Gondwana, while in the far north, the ocean must have been covered with miles upon miles of broad sea ice, Montanez said.

Then carbon dioxide gas, emerging into the atmosphere from minerals and weathered rocks, volcanoes and carbonates in the seas, began to increase sporadically, and what had been a continuously frozen continent began to thaw.

According to Lee R. Kump, professor of geosciences at Pennsylvania State University, the report from Montanez and her colleagues "makes a pretty compelling case" for that long-ago change from an ice-bound world to continuing episodes of hot, dry global climate.

In an interview, Kump noted that although some skeptics have questioned whether carbon dioxide is indeed the major "driver" in any global warming scenario, the evidence is now strong that it is. For the past 400,000 years, he said, atmospheric levels of the gas have ranged naturally from 180 to 280 parts per million, while today's "human perturbed" level has already risen to about 370 parts per million.

"This study," he said of the report in Science, "clearly shows how a period of strong warming came at the end of a long, intensely glacial period — and we're breaking out of another million-year glacial period again right now."